The present invention relates to an analog-to-digital (A/D) converter, more particularly, to an integration-type A/D converter.
In general, A/D converters are divided into successive comparison-types and integration-types. A successive comparison-type A/D converter, which includes a digital-to-analog (D/A) converter, compares an analog voltage with successive outputs of the D/A converter. This successive comparison-type converter features a relatively high conversion rate, but is low in accuracy and high in manufacturing cost. On the other hand, an integration-type A/D converter, which uses a constant slope approach, is high in accuracy and low in manufacturing cost, but features a relatively low conversion rate.
A prior art integration-type A/D converter (see: IEEE Journal of Solid-State Circuits, Vol. SC-15, No. 1, February 1980) comprises a charging switch, a capacitor, a constant current source, a discharging switch, a detection circuit, and a timer-counter. During the charging mode, the charging and discharging switches are turned on and off, respectively, to charge the capacitor and disable the constant current source. Next, during the discharging mode, the charging and discharging switches are turned off and on, respectively, to activate the constant current source and, accordingly, discharge the capacitor. Simultaneously, the timer-counter is initiated. In this case, the voltage of the capacitor is monitored by the detection circuit which, in turn, detects whether or not the voltage reaches a predetermined value (threshold value). At the threshold crossing, the detection circuit stops the timer-counter. This value of the timer-counter is used as a digital value. More precisely, the ratio of this value to a reference value is used as a digital value.
In the above-mentioned prior art A/D converter in order to enhance the accuracy, or in other words, in order to enlarge the time-period of the discharging operation, the capacitance of the capacitor needs to be so large that the capacitor must be an external component. Therefore, an on-chip A/D converter needs more external pins, reducing the manufacturing yield and, accordingly, increasing the manufacturing cost. In addition, high accuracy cannot be expected, since the noise generated from the external pins increases. Further, the provision of such an external capacitor is disadvantageous when the A/D converter is incorporated into a signal processor or a one-chip microcomputer.